As one skilled in this technology appreciates, the gas turbine power plant typically incorporates fans and compressor blades for imparting compression to the intake air (straight jet engines do not incorporate fans) which is then delivered to the combustor where fuel is combusted to add heat thereto. Hence, this engine working medium is accelerated before being delivered to the turbine which is mechanically connected to the fan/compressor so that the energy extracted from the engine working medium is converted to power the compressor fan/blades to pressurize the intake air and develop propulsive thrust. The fan blade, which essentially is a compressor blade that is typically mounted upstream of the smaller compressor blades, not only serves to pressurize the intake air, it also develops thrust which is added to the overall thrust developed by the engine.
As will become apparent from the description to follow, this invention is particularly efficacious for use in an IBR of a gas turbine engine, but also has potential use for airfoils that are utilized in a non IBR configuration. The following discussion relates to the IBR configuration.
Modern day fan blades are generally highly sophisticated so as to achieve high pressure ratios across the stage(s) of fan blades which require swept airfoils and contoured blade shaping to attain the necessary superior aerodynamics. The backward sweep of the airfoil leading edge typically serves to reduce shock losses and noise generation while the forward sweep is typically incorporated to enhance the aerodynamics of the airfoil.
In addition to the aerodynamics of the airfoil, be it a fan, compressor or turbine blade, a concern to the designer and engine user is the stress concentration of the blade. During operation the blades and disk rotate and generate substantial centrifugal forces which are carried by the disk. The designer of the IBR must assure that the steady tensile stress and the alternating stress are maintained within their limits throughout the entire IBR configuration for the entire operating envelope of the power plant. What is not in the control of the designer is localized stress concentration that is occasioned by blade damage due to foreign object ingestion. Ingested objects, such as grit or sand from the runways, small birds, ice, etc. during normal operation may cause chips or nicks in the blades and most commonly in the leading edge. Obviously, because the IBR is a complicated hardware requiring special materials and expensive manufacturing, it is extremely costly to discard an IBR whenever the blade becomes damaged. Since the IBR is an integral unit of the blades and disks, obviously the removal of the blade for repair purposes is non-existing.
While the teachings disclosed in U.S. Pat. No. 5,725,354 granted to Wadia, et al on Mar. 10, 1998 entitled FORWARD SWEPT FAN BLADE, relates to a separate leading edge, the present invention differs not only in the construction thereof, but also the design philosophy. To best understand the difference between these two concepts, one should appreciate the teachings in the Wadia et al reference, supra, where the swept portion of a IBR fan blade is made from a separate composite piece that fits into the portion of the airfoil of the fan blade so as to attain the forward sweep. As is true in all fan blades, the centrifugal loads generated by the disk create predominantly radial centrifugal forces in the blades which causes a steady tensile stress. In addition, the airflow over the blades induces vibration in the airfoil and creates alternating stresses. These stresses, i.e., steady tensile stress and alternating stresses are not uniform and vary over the length, width and thickness of the blade. This situation is exacerbated by indications on the airfoil such as nicks, chips, cracks and the like caused by sand, dirt, bird ingestion, etc. that causes stress concentration. The problem is more complicated when the leading edge is significantly swept forward. While this innovation provides aerodynamic benefits, it changes the center of gravity of the blade and hence, complicates the stress design of the airfoil. Hence, the purpose of the teachings of the '354 patent, supra, is to provide a design configuration that improves the stress problem by reducing the steady stress along the leading edge of the airfoil which would otherwise occur from effecting forward sweep.
Of significance, the present invention differs from the '354 patent and other heretofore known prior art designs, by 1) providing a removable leading edge that can be fabricated from the same or different materials and 2) designing the leading edge as an independent load carrying member where it includes an airfoil portion and an attachment portion (root), such that the loads on the airfoil are transmitted from the airfoil through the root of the leading edge into the disk. Hence, the disk, whether an IBR or a non-IBR fan must be designed to accommodate the root of the leading edge for attachment thereto.